Investors yanked $12.3 billion out of U.S. equity funds last week, the biggest outflow in more than five years, according to Michael Hartnett, chief investment strategist at Bank of America Corp.'s Merrill Lynch. The outflow marked the first time in eight weeks that investors yanked money out of U.S. stock funds.

Mr. Hartnett points out the bulk of that amount — $10 billion — came from the ETF market’s largest fund, the SPDR S&P 500 (SPY). ETFs often attract a hefty dose of “hot” money that flows in and out from traders and hedge funds.

There has been a “flight to cash as retail investors dump risk assets,” Mr. Hartnett said in a note to clients on Friday, pointing out a combined $20 billion flew out of stocks and bonds last week, while money-market funds saw an influx of new cash.

The question now is whether the big outflows across asset classes will mark the beginning of a new trend, especially as concerns about the Fed’s next move ramp up. Fed officials reaffirmed their plans earlier this week to try winding down its bond-buying program, but left investors wondering when or how aggressively they would move. The uncertainty has weighed on stock prices and pushed bond yields higher.

The Dow notched a second straight day of gains on Friday and finished above 15000. But the blue-chip average is riding a three-week losing streak, its longest since a four-week stretch that ended in the middle of November.

Judging by the latest fund-flow data, investors are selling now and asking questions later.